I just went to put the door on the oven and cover it up as it looks like it might rain and i noticed a bit of moisture seeping from the sidewalls.It did actually hit 170` Celcius right at the end.

FYI, when I was curing I did not have any water visible or wet anything around the base. And did not notice any condensation after the initial firings etc. However I did run some all day fires, and fires over night from when I got home from work and put the last log on the fire at midnight. In the morning if the tarp was touching the dome, it would be saturated.

This behaviour lasted two weeks after expected initial curing. The tarp only touched the top of the dome if not tied up enough. So I think the migration of water in volume might be in vapour form/humidity and is likely going to be towards the top of the dome/arched roof.

Gudday
Perhaps the language should change?
Drying and "tempering " the oven to take high temps that we expect might change?
We seem to use the same "term" for both.
There are a lot of cracked ovens out there....yep it's great to except it as a badge of honour... But does it have to be?
No.., I'm not the expert here....but.
Low and slow seems to be the way most folk start....but all of a sudden, the temps rise with the same amount of heat source ,the oven starts to dry out..... And it's cracks?
Why???
Regards dave

My theory is that the top of the dome gets hot first, particularly because of rising heat and flame impingement. This results in the top drying first. Because a considerable amount of energy is required to convert the water to steam, the temperature in the refractory that still contains water is dramatically lower. An infrared temp gun taking readings bears this out. The temperature at the crown can be some 150 C higher than the walls at the bottom and the floor at the perimeter. This big difference in temperature leads in turn to uneven expansion rates which I believe are responsible for the cracks. Unfortunately, just as the top of the dome begins to dry, so the efficiency of the oven and the combustion starts to run away. You think ah hah it's finally beginning to fire properly. This of course simply leads the temperature of the top to rise even more creating greater uneven expansion. I think the remedy is to heat the oven slowly and avoid direct flame impingement on to the top of the dome as well as spreading the coals out to the outside of the floor perimeter.

What im getting from all this is keep it low and slow,sounds like great advice to me.
I totally agree that the dome refractory will have dried a lot earlier than the sides and this could catch the unwary out by increasing curing temps.
Cheers,John.

Well to my surprise this morning i discovered 3 cracks along the sidewalls,not nice!
They are almost symmetrical,one on each wall equidistant from each other.They are not massive and they continue through the vermi-mix insulation.I checked the dome internally and there are no cracks inside at all.
Seems that the vermi mix was still holding a lot of moisture and forcing its way outwardly,hence the moisture seepage from yesterday.

A question to the veterans,
How soon could i be putting on the final vermicrete mix?
It should be known that im still curing/drying the oven but my temps are still quite low.Internal ambient temp. is 250-300 C using heat beads.I've been extra conservative since i noticed some cracks externally.
Im convinced there is no more moisture to contend with so would it be a good idea to start with the vermi mix.? Also some suggestions for the ratio for this mix over the dome.I was going to use 6:1.

The usual mix for under the oven is around 6:1. This fairly rich brew is required for adequate strength to support the oven. You could use the same brew over the oven however, as it does not require any strength except to hold itself together so you have something firm enough to render over, it is better to make it much leaner. The more cement you add to the brew the more you reduce it's insulation value. I use 10:1 for the sides and over the dome, some go as lean as 13:1, but I find that more difficult to apply. For every 10 litres of vermiculite I also throw in a handful of clay, which helps make the mix stick a bit better.

I like to give it a week to allow the sun and wind to remove some of the water before fires. You have about three times more water in the mix than is required for hydration so there is little point in covering it to retain moisture for strength.

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